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The thermal stability and decomposition pathway of formamidinium iodide (FAI, HC(NH 2 ) 2 I) in contact with NiO and TiO 2 are investigated by combined experimental studies and density functional theory (DFT) calculations. Based on the decomposition temperature, we find that the stability decreases as FAI ∼ FAI + TiO 2 > FAI + NiO. Moreover, FAPbI 3 in contact with NiO and TiO 2 shows similar thermal stability behaviour to FAI. The bulk decomposition of FAI occurs via the formation of sym -triazine, and can also produce HCN, and NH 4 I at ∼280 °C, which further decomposes to NH 3 and HI above 300 °C. When FAI comes into contact with NiO, the interfacial reaction triggers decomposition at a much lower temperature (∼200 °C), resulting in the formation of NiI 2 as the solid product while releasing NH 3 and H 2 O into the gas phase; sym -triazine and HCN are observed near the FAI bulk decomposition temperature. In contrast, when FAI comes into contact with TiO 2 , the decomposition temperature is similar to bulk FAI; however, HCN is released at a lower temperature (∼260 °C) compared to sym -triazine. The difference in the degradation behavior of FAI with NiO and TiO 2 is elucidated using DFT calculations. Our results show that the interfacial reaction between the organic component of perovskite material and NiO occurs similarly for MA and FA, which thereby can induce device instability. 

Abstract New light is shed on the previously known perovskite material, Cs₂Au₂I₆, as a potential active material for high‐efficiency thin‐film Pb‐free photovoltaic cells. First‐principles calculations demonstrate that Cs₂Au₂I₆has an optimal band gap that is close to the Shockley–Queisser value. The band gap size is governed by intermediate band formation. Charge disproportionation on Au makes Cs₂Au₂I₆a double‐perovskite material, although it is stoichiometrically a single perovskite. In contrast to most previously discussed double perovskites, Cs₂Au₂I₆has a direct‐band‐gap feature, and optical simulation predicts that a very thin layer of active material is sufficient to achieve a high photoconversion efficiency using a polycrystalline film layer. The already confirmed synthesizability of this material, coupled with the state‐of‐the‐art multiscale simulations connecting from the material to the device, strongly suggests that Cs₂Au₂I₆will serve as the active material in highly efficient, nontoxic, and thin‐film perovskite solar cells in the very near future.